1. Technical Field of the Invention
The embodiments of the invention relate generally to disk drives and, more particularly, to providing a guided search for a target to set up a disk drive.
2. Description of Related Art
Varieties of memory storage devices, such as magnetic disk drives, are available to store data and are used to provide data storage for a host device, either directly, or through a network. Those networks may be a storage area network (SAN) or a network attached storage (NAS). Typical host devices include stand alone computer systems such as a desktop or laptop computer, enterprise storage devices such as servers, storage arrays such as a redundant array of independent disk (RAID) arrays, storage routers, storage switches and storage directors, and other consumer devices such as video game systems and digital video recorders. These devices generally provide high storage capacity in a cost effective manner.
Within some of these memory storage devices, there may be performance issues associated when performing timing recovery of a signal that is read from the disk. That is, appropriate digital sampling is performed for accurate and effective recovery of the information from signals read from the disk. For example, if improper or inadequate digital sampling is performed on the signal that is read from the disk, then some of the data recovered may be inaccurate or not recovered at all. In some instances, the system may try to re-read the data (e.g., via a re-try) requiring additional time. Furthermore, in devices where energy consumption is a concern (e.g., small hand held devices and/or battery operated devices), performing more data accesses than necessary may reduce the operational capabilities of the battery.
In order to continually improve the data read capabilities of storage devices, such as hard disk drives (HDD), various sampling techniques may be implemented or enhanced to read, sample and recover the stored data. These sampling techniques may also be used to monitor markers placed on the disk, which markers are used to control the synchronization and/or speed of the disk drive. In reading the data from a HDD, the disk drive is brought up to operational speed and then the data is read from a given sector of a storage disk. In some instances where battery life is a concern, to conserve battery life, the HDD is actively on as little as possible while still maintaining a low data error rate.
In a typical HDD, a channel for reading the data from a disk is referred to as a read channel. The read channel may employ one of a variety of techniques to read signals from the disk and convert the read signals into 0s and 1s that correspond to the data that was originally stored on the disk. If the HDD uses a magnetic disk as the medium to store the written data, the data is retained on the disk as aligned magnetic fields. The magnetic fields may have horizontal or vertical alignment on the medium depending on which alignment scheme is used. Generally, each field alignment is given +1 or −1 in bi-polar NRZ representation corresponding to opposite magnetization direction, which can be mapped to digital 1s and 0s respectively.
In recovering the stored data from a magnetic disk, a magnetic pick-up head reads the field alignment as the head traverses over the disk. The head sends the picked-up signal to the read channel circuitry of the HDD in order to sample and detect the read signal from the head and generate a detected signal which, corresponds to the original stored signal, if done properly.
A variety of read channel technologies are available for detecting the read signal from the disk. Common techniques are run-length limited coding and peak detection. More recently, partial-response, maximum likelihood (PRML) detection has been implemented in read channels of magnetic HDDs to address recovery of data from high density disks. PRML is a technique that uses sampling that allows a sequence of bits to be detected, instead of individual bits, and increases the detection capabilities on higher capacity HDDs. PRML detection is further described in 1) “A PRML System for Digital Magnetic Recording” by Roy D. Cideciyan et al., IEEE Journal on Selected Areas in Communications, Vol. 10, No. 1, January 1992, pages 38-56; and in 2). “PRML detection boosts hard-disk drive capacity” by Kevin D. Fisher et al., The Practical Engineer, IEEE Spectrum, November 1996, pages 70-76.
In implementing a PRML system, a common technique is to equalize the read channel response so that some form of detection, such as Viterbi Algorithm, may be applied to detect the signal. In order to obtain the adaptive equalization for the read channel, partial response (PR) targets are selected that closely matches the channel response. Equalization techniques are described in 3) “Equalization for Maximum Likelihood Detectors” by Jaekyun Moon et al., IEEE Transactions on Magnetics, Vol. 31, No. 2, March 1995, pages 1083-1088; and in 4) “Equalization and Detection for Nonlinear Recording Channels with Correlated Noise” by Nick M. Zayed et al., IEEE Transactions on Magnetics, Vol. 35, No. 5, September 1999, pages 2295-2297. Generally, a Finite Impulse Response (FIR) filter is employed in which the coefficients of the filter may be changed for a given target response.
In order to obtain the proper equalization in the read channel, one technique is to fix a target response for the read channel that is desirable and, then, find optimal FIR coefficients corresponding to the target when designing a detector. However, this technique is limiting since the target is fixed and cannot easily accommodate for a variety of channels (with different drive head and media combinations). Another technique is to search for the right target from a pool of possible targets by formulation based on approximation of bit error rate (BER) to derive the PR value and then searching for the optimal target from the pool. Although these techniques may obtain acceptable PR targets, the techniques are not as flexible and/or involve significant computation/search to obtain the optimal target.
Accordingly, there is a need for another technique to obtain PR targets.